Cutting Tool and Cutting Insert Having Exactly Three Cutting Portions Therefor

ABSTRACT

In a cutting tool having an indexable cutting insert with three cutting portions, the cutting insert is removably securable to an insert holder by means of a fastener. The cutting insert has two opposing end surfaces with a peripheral side surface extending therebetween. At least one end surface has a central boss protruding therefrom with a raised support surface, and the peripheral side surface has three abutment recesses. In an end view of the cutting insert, each of the three abutment recesses is visible and located inside a first imaginary circle circumscribing the visible central boss. A holding portion of the insert holder has a seating surface with at least one protuberance protruding therefrom. The support surface is in clamping contact with the seating surface, and exactly two of the three abutment recesses are engaged with the at least one protuberance.

RELATED APPLICATIONS

This is a Continuation of U.S. patent application Ser. No. 13/733,717,filed Jan. 3, 2013, now U.S. Pat. No. ______, whose contents areincorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to metal cutting tools for use in grooveturning and groove milling operations having an indexable cutting insertwith a plurality of cutting portions.

BACKGROUND OF THE INVENTION

Within the field of metal cutting tools used in groove turning andgroove milling operations, cutting inserts removably securable in aninsert holder have long since provided a way of performing a cuttingoperation with a suitably hard material, i.e. cemented carbide, in thevicinity of the cutting edge, where the insert holder, manufactured froma less hard material, is reusable following the disposal of a worn ordamaged cutting insert.

This type of cutting tool has been further developed to utilizeindexable cutting inserts with an increased number of cutting edges,giving economic benefits from providing an increased number of cuttingoperations per cutting insert. This type of cutting tool has also beendeveloped to provide a more efficient means of securing the indexableinsert to the insert holder.

U.S. Pat. No. 4,461,602 discloses a milling cutter consisting of ashaft, a cutter plate and a screw bolt by means of which the cutterplate is detachably fastened at the front end of the shaft. The latterhas a recess in the front face thereof. The recess is defined by threeplanar side faces leaving a peripheral gap between each pair thereof.The cutter plate substantially formed triangularly is fitted in therecess for being inmovable in a radial plane. Protrusions of the cutterplate protrude through the gaps and are provided with a cutting edgerespectively.

U.S. Pat. No. 5,607,263 discloses a cutting plate having a disk-shapedbody with three teeth and a single integrally formed projection. Theholder has a front-face recess which is encompassed all around by a wallof the holder. As shown in FIGS. 5 and 6 of U.S. Pat. No. 5,607,263, theprojection has a coupling surface with the contour of a triangularconvex polygon and the recess has a counter-surface with the contour ofa triangular concave polygon, and in a loaded state three contact zonesare formed between the coupling surface and the counter-surface.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided an indexablecutting insert comprising two opposing end surfaces with a peripheralside surface and a central axis extending therebetween,

a median plane located between the two opposing end surfaces,

exactly three cutting portions, each cutting portion having a majorcutting edge extending from one end surface to the other end surface,

exactly three abutment recesses in the peripheral side surface, and

a central boss protruding from at least one of the two opposing endsurfaces,

wherein each central boss has a raised support surface located axiallyfurther away from the median plane than any point on the three majorcutting edges located on the same side of the median plane, and

wherein in an end view of the cutting insert each of the three abutmentrecesses is visible and located inside a first imaginary circlecircumscribing the central boss.

Also in accordance with the present invention, there is provided acutting tool comprising an insert holder, and at least one cuttinginsert of the sort described above retained therein.

The insert holder has a holding portion extending along a longitudinaltool axis, the holding portion having a seating surface transverse tothe longitudinal tool axis, with at least one protuberance protrudingfrom the seating surface,

the at least one cutting insert being removably securable to the insertholder by means of a fastener,

wherein exactly one cutting portion of each cutting insert is operative,

wherein one support surface of each cutting insert is in clampingcontact with the seating surface, and

wherein exactly two of the three abutment recesses of each cuttinginsert are engaged with the at least one protuberance.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding, the invention will now be described, by wayof example only, with reference to the accompanying drawings in whichchain-dash lines represent cut-off boundaries for partial views of amember and in which:

FIG. 1 is a perspective view of a cutting insert in accordance with someembodiments of the present invention;

FIG. 2 is an end view of the cutting insert shown in FIG. 1;

FIG. 3 is a cross-sectional view of the cutting insert shown in FIG. 2,taken along the line III-III;

FIG. 4 is a cross-sectional view of the cutting insert shown in FIG. 3,taken along the line IV-IV;

FIG. 5 is a detailed view of the cutting insert shown in FIG. 3;

FIG. 6 is an exploded perspective view of a cutting tool in accordancewith some embodiments of the present invention;

FIG. 7 is an end view of the cutting tool shown in FIG. 6, with thefastener removed;

FIG. 8 is a perspective view of an insert holder in accordance with someembodiments of the present invention; and

FIG. 9 is an end view of the insert holder shown in FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

Attention is first drawn to FIGS. 1 to 5, showing an indexable cuttinginsert 20 which may be manufactured by form pressing and sintering acemented carbide, such as tungsten carbide, and may be coated oruncoated.

According to the present invention, the cutting insert 20 has twoopposing end surfaces 22 with a peripheral side surface 24 and a centralaxis A1 extending therebetween, and exactly three cutting portions 26.

In some embodiments of the present invention, the cutting insert 20 maybe indexable about the central axis A1.

According to the present invention, the peripheral side surface 24 hasexactly three abutment recesses 28, and each of the three cuttingportions 26 has a major cutting edge 30 extending from one end surface22 to the other end surface 22.

In some embodiments of the present invention, as shown in FIGS. 1 and 4,each of the three major cutting edges 30 may be formed by theintersection of a rake surface 32 and a clearance surface 34, and therake and clearance surfaces 32, 34 may be located on the peripheral sidesurface 24. As seen in FIG. 4, at least a portion of each of abutmentrecess 28 may be formed within a peripheral recess 33 which includes therake surface 32 of an associated major cutting edge 30.

Also, in some embodiments of the present invention, the three cuttingportions 26 may be circumferentially equally spaced about the centralaxis A1 alternating with the three abutment recesses 28.

Further, in some embodiments of the present invention, the three cuttingportions 26 may be circumferentially and radially spaced apart from thethree abutment recesses 28.

Yet further, in some embodiments of the present invention, all of thethree cutting portions 26 may be identical.

Yet still further, in some embodiments of the present invention, all ofthe three abutment recesses 28 may be identical.

It should be appreciated that use of the terms “radially” and “axially”throughout the description and claims refers to the positioning ofelements of the cutting insert 20 with respect to the central axis A1.

As shown in FIG. 2, the cutting insert 20 may exhibit rotationalsymmetry about the central axis A1.

In some embodiments of the present invention, as shown in FIG. 2, eachof the three cutting portions 26 may have a radial plane P1 containingthe central axis A1 and a first point N1 on its respective major cuttingedge 30. As best seen from the end view of FIG. 2 and FIG. 3 showing thecross-section of FIG. 2 along the radial plane P1, at least a portion ofthe rake surface 32 of each cutting portion 26 is located on the sameside of its respective radial plane P1 as the adjacent clearance surface34.

Also, in some embodiments of the present invention, as shown in FIG. 2,each radial plane P1 may form an acute internal clearance angle α with astraight imaginary clearance line L_(C) tangential to the clearancesurface 34 immediately adjacent its respective first point N1, and theacute clearance angle α may have a value equal to or greater than 75degrees, i.e. α≧75°.

Configuring the cutting insert 20 to have clearance angles α equal to orgreater than 75 degrees advantageously provides robust support for theoperative cutting edge 30 during internal groove turning operations.

It should be appreciated that use of the terms “internal angle” and“external angle” throughout the description and claims refers to anangle between two planar and/or linear components as measured internaland external to the member on which these components are formed,respectively.

According to the present invention, as shown in FIGS. 3 and 5, thecutting insert 20 has a median plane M located between the two opposingend surfaces 22, and a central boss 36 protruding from at least one ofthe two opposing end surfaces 22. Each central boss 36 has a raisedsupport surface 38 located axially further away from the median plane Mthan any point on the three major cutting edges 30 located on the sameside of the median plane M.

Configuring the cutting insert 20 with raised support surfaces 38advantageously enables modification of the three cutting portions 26 toachieve multiple product variants without modifying the support surfaces38.

In some embodiments of the present invention, each of the three cuttingportions 26 may be radially spaced apart from the at least one centralboss 36.

Also, in some embodiments of the present invention, as shown in FIG. 2,each central boss 36 may have three second points N2 contained in thethree radial planes P1, and each second point N2 may be located theshortest distance between its respective central boss 36 and itsrespective first point N1.

Further, in some embodiments of the present invention, as shown in FIG.2, each second point N2 may be located on a curved portion 76 of itsrespective central boss 36, and each curved portion 76 may be outwardlyconvex in relation to the central axis A1.

Configuring each curved portion 76 to be outwardly convex in relation tothe central axis A1 advantageously enables cutting depths of internalgrooving operations to be optimized.

Yet further, in some embodiments of the present invention, each of thethree cutting portions 26 may have two minor cutting edges 40, eachminor cutting edge 40 extending away from its associated major cuttingedge 30 in a direction towards the median plane M.

As shown in FIGS. 3 and 5, both end surfaces 22 may have a central boss36, such that the cutting insert 20 has a total of two central bosses36.

In some embodiments of the present invention, as shown in FIG. 3, eachsupport surface 38 may be substantially planar and perpendicular to thecentral axis A1.

Configuring each support surface 38 to be planar and perpendicular tothe central axis A1 advantageously facilitates accurate and repeatableindexing of the cutting insert 20.

Also, in some embodiments of the present invention, as shown in FIGS. 3and 5, each support surface 38 may be the axially outermost surface ofits associated end surface 22.

As shown in FIG. 3, the median plane M may be perpendicular to thecentral axis A1.

In some embodiments of the present invention, the median plane M mayintersect the peripheral side surface 24 along the whole of theperimeter of the peripheral side surface 24.

Also, in some embodiments of the present invention, as shown in FIG. 3,the cutting insert 20 may exhibit mirror symmetry about the median planeM. In such embodiments, the cutting insert 20 is reversible and canadvantageously be suitable for both left and right-handed cutting tools.

According to the present invention, in an end view of the cutting insert20, as shown in FIG. 2, each of the three abutment recesses 28 isvisible and at least partially located inside a first imaginary circleC1 circumscribing the visible central boss 36. The first imaginarycircle C1 has a first diameter D1.

Configuring the cutting insert 20 with the three abutment recesses 28inside the first imaginary circle C1 advantageously enables the cuttinginsert 20 to be stably secured to a mating body with an efficient use ofspace.

In some embodiments of the present invention, each of the three abutmentrecesses 28 may be visible in both end views of the cutting insert 20.

Also, in some embodiments of the present invention, each of the threeabutment recesses 28 may intersect each central boss 36.

In alternative embodiments of the present invention (not shown), eachcentral boss may comprise three spaced apart protrusions having coplanarsupport sub-surfaces.

As shown in FIG. 2, the three major cutting edges 30 may define a secondimaginary circle C2 having a second diameter D2 which circumscribes theperipheral side surface 24.

In some embodiments of the present invention, the center of the secondimaginary circle C2 may be coincident with the central axis A1.

Also, in some embodiments of the present invention, the first diameterD1 may be equal to or less than two thirds of the second diameter D2,i.e. D1≦⅔D2.

Further, in some embodiments of the present invention, each curvedportion 76 may have a radius equal to or smaller than the radius of thesecond imaginary circle C2.

Configuring the radius of each curved portion 76 to be equal to orsmaller than the radius of the second imaginary circle C2 advantageouslyenables cutting depths of internal grooving operations in small diameterbores to be optimized.

In an end view of the cutting insert 20, as shown in FIG. 2, a thirdimaginary circle C3 may contain the visible central boss 36 and exactlytwo of the three cutting portions 26.

In some embodiments of the present invention, the second and thirdimaginary circles C2, C3 may have the same second diameter D2, and anoffset distance O between the centers of the second and third imaginarycircles C2, C3 may be equal to or greater than a quarter of their seconddiameter D2, i.e. O≧D2/4.

In an end view of the cutting insert 20, as shown in FIGS. 2 and 4, eachof the three abutment recesses 28 may have a pair of spaced apartabutment surfaces 44 a, 44 b forming a V-shape.

In some embodiments of the present invention, each pair of abutmentsurfaces 44 a, 44 b may be spaced apart by a curved joining surface 46,and each joining surface 46 may contain the radially innermost point ofits associated abutment recess 28.

Also, in some embodiments of the present invention, each abutmentsurface 44 a, 44 b may contain a straight imaginary abutment line L_(A)parallel to the central axis A1.

Further, in some embodiments of the present invention, each abutmentsurface 44 a, 44 b may be substantially planar, and in a cross-sectionof the cutting insert 20 taken in a plane perpendicular to the centralaxis A1, as shown in FIG. 4, each pair of abutment surfaces 44 a, 44 bmay form an external recess angle β having a value between 60 and 120degrees, i.e. 60°<β<120°.

Yet further, in some embodiments of the present invention, the recessangle β between each pair of abutment surfaces 44 a, 44 b may have avalue between 60 and 120 degrees in any cross-section of the cuttinginsert 20 taken in a plane perpendicular to the central axis A1.

In some embodiments of the present invention, each end surface 22 mayinclude a lower surface 48 extending radially inwardly from each of thethree cutting portions 26.

As shown in FIG. 5, each lower surface 48 may be axially closer to themedian plane M than a portion of the major cutting edge 30 of itsassociated cutting portion 26.

Also, as shown in FIG. 5, each lower surface 48 may be substantiallyplanar and perpendicular to the central axis A1, and each lower surface48 may define a lower plane P2 which intersects the major cutting edge30 of its associated cutting portion 26.

Attention is now drawn to FIGS. 6 and 7 showing a cutting tool 50according to the present invention, comprising an insert holder 52 andat least one cutting insert 20.

The at least one cutting insert 20 is removably secured to the insertholder 52 by means of a fastener 54.

As shown in FIGS. 8 and 9, the insert holder 52 has a holding portion 56extending along a longitudinal tool axis A2, and the holding portion 56has a seating surface 58 transverse to the longitudinal tool axis A2,with at least one protuberance 60 a, 60 b protruding from the seatingsurface 58.

In some embodiments of the present invention, the seating surface 58 maybe substantially planar and perpendicular to the longitudinal tool axisA2.

According to the present invention, as shown in FIG. 7, exactly onecutting portion 26 of each cutting insert 20 is operative, one supportsurface 38 of each cutting insert 20 is in clamping contact with theseating surface 58, and exactly two of the three abutment recesses 28 ofeach cutting insert 20 are engaged with the at least one protuberance 60a, 60 b.

In some embodiments of the present invention, as shown in FIG. 7,exactly two spaced apart corner portions 42 a, 42 b of the at least oneprotuberance 60 a, 60 b may be located inside the first imaginary circleC1 of each cutting insert 20.

Also, in some embodiments of the present invention, as shown in FIGS. 2and 7, each cutting insert 20 may comprise a central bore 62 extendingcoaxially with its central axis A1.

Further, in some embodiments of the present invention, the central bore62 may be visible in both end views of the cutting insert 20.

Yet further, in some embodiments of the present invention, as shown inFIG. 6, the fastener 54 may be in the form of a clamping screw 64extending through the cutting insert's central bore 62 and engaging athreaded bore 66 in the seating surface 58 of the holding portion 56.

As shown in FIG. 7, the rake surfaces 32 of each cutting insert 20 maygenerally face in a first circumferential direction d1 about the centralaxis A1, and the exactly two abutment recesses 28 of each cutting insert20 engaging with the at least one protuberance 60 a, 60 b may be thefirst and second closest abutment recesses 28 a, 28 b to its operativecutting portion 26 a in an opposite second circumferential direction d2about its central axis A1. Meanwhile, the closest abutment recess 28 cin the first circumferential direction d1 about the central axis A1remains unengaged.

Configuring the cutting tool 50 such that the first closest abutmentrecess 28 a to its operative cutting portion 26 a in the secondcircumferential direction d2 engages with the at least one protuberance60 a, 60 b can advantageously support the operative cutting portion 26 aand absorb operational cutting forces.

In some embodiments of the present invention, both of the abutmentsurfaces 44 a, 44 b associated with the first closest abutment recess 28a to its operative cutting portion 26 a in the second circumferentialdirection d2 may be in clamping contact with the at least oneprotuberance 60 a, 60 b, and only one of the abutment surfaces 44 aassociated with the second closest abutment recess 28 b to its operativecutting portion 26 a in the second circumferential direction d2 may bein clamping contact with the at least one protuberance 60 a, 60 b.

Also, in some embodiments of the present invention, the two abutmentsurfaces 44 a, 44 b associated with the first closest abutment recess 28a to its operative cutting portion 26 a in the second circumferentialdirection d2 may be in clamping contact with corresponding first andsecond reaction walls 68, 70 of the at least one protuberance 60 a, 60b, and the single abutment surface 44 a associated with the secondclosest abutment recess 28 b to its operative cutting portion 26 a inthe second circumferential direction d2 may be in clamping contact witha corresponding third reaction wall 72 of the at least one protuberance60 a, 60 b, and each of the first, second and third reaction walls 68,70, 72 may contain a straight imaginary reaction line L_(R) parallel tothe longitudinal tool axis A2.

As shown in FIG. 7, the single abutment surface 44 a in clamping contactwith the third reaction wall 72 may generally face in the secondcircumferential direction d2 about its central axis A1.

In some embodiments of the present invention, each threaded bore 66 maybe eccentric in relation to its respective central bore 62. In suchembodiments, tightening of the clamping screw 64 within its threadedbore 66 urges each cutting insert 20 in a transverse direction withrespect to the longitudinal tool axis A2, thus ensuring clamping contactbetween three of its abutment surfaces 44 a, 44 b and threecorresponding reaction walls 68, 70, 72.

Also, in some embodiments of the present invention, as shown in FIGS. 6and 7, exactly one cutting insert 20 may be secured to the insert holder52, exactly two spaced apart protuberances 60 a, 60 b may protrude fromthe seating surface 58, and the first and second reaction walls 68, 70may be located on a first protuberance 60 a and the third reaction wall72 may be located on a second protuberance 60 b. In such embodiments,the first protuberance 60 a has a first corner portion 42 a and thesecond protuberance 60 b has a second corner portion 42 b.

Configuring the insert holder 52 with exactly two spaced apartprotuberances 60 a, 60 b advantageously enables the single cuttinginsert 20 to be secured to the insert holder 52 with an efficient use ofspace.

In some embodiments of the present invention, having exactly one cuttinginsert 20 secured to the insert holder 52, the holding portion 56 may beelongated and longitudinally extend away from a shank portion 74.

In an end view of the cutting tool 50, as shown in FIG. 7, the secondimaginary circle C2 may contain the entire holding portion 56.

Also, in an end view of the cutting tool 50, as shown in FIG. 7, thethird imaginary circle C3 may contain the entire holding portion 56.

As the offset distance O between the centers of the second and thirdimaginary circles C2, C3 may be equal to or greater than a quarter oftheir second diameter D2, i.e. O≧D2/4, the cutting tool 50 canadvantageously be used to perform internal groove turning operations atcutting depths equal to or greater than a quarter of the bore diameterof the workpiece within which the holding portion 56 extends.

In alternative embodiments of the present invention (not shown), thecutting tool may have a plurality of cutting inserts removably securedto the seating surface of the insert holder, and the cutting tool may berotatable about the longitudinal tool axis and used in groove millingoperations.

Although the present invention has been described to a certain degree ofparticularity, it should be understood that various alterations andmodifications could be made without departing from the spirit or scopeof the invention as hereinafter claimed.

What is claimed is:
 1. An indexable cutting insert (20) comprising: twoopposing end surfaces (22) with a peripheral side surface (24) and acentral axis (A1) extending therebetween, a median plane (M) locatedbetween the two opposing end surfaces (22), exactly three cuttingportions (26), each cutting portion (26) having a major cutting edge(30) extending from one end surface (22) to the other end surface (22),and a central boss (36) protruding from each of the two opposing endsurfaces (22), wherein each central boss (36) has a raised supportsurface (38) located axially further away from the median plane (M) thanany point on the three major cutting edges (30) located on the same sideof the median plane (M), wherein each of the three major cutting edges(30) is formed by the intersection of a rake surface (32) and aclearance surface (34), and wherein the rake and clearance surfaces (32,34) are located on the peripheral side surface (24), wherein each of thethree cutting portions (26) has a radial plane (P1) containing thecentral axis (A1) and a first point (N1) on its respective major cuttingedge (30), and wherein in an end view of the cutting insert (20), atleast a portion of the rake surface (32) of each cutting portion (26) islocated on the same side of its respective radial plane (P1) as theadjacent clearance surface (34).
 2. The cutting insert (20) according toclaim 1, wherein each support surface (38) is substantially planar andperpendicular to the central axis (A1), and wherein each support surface(38) is the axially outermost surface of its associated end surface(22).
 3. The cutting insert (20) according to claim 1, wherein themedian plane (M) intersects the peripheral side surface (24) along thewhole of the perimeter of the peripheral side surface (24), and whereinthe median plane (M) is perpendicular to the central axis (A1).
 4. Thecutting insert (20) according to claim 1, wherein the cutting insert(20) exhibits minor symmetry about the median plane (M).
 5. The cuttinginsert (20) according to claim 1, wherein the cutting insert (20) isindexable about the central axis (A1), and wherein the cutting insert(20) exhibits rotational symmetry about the central axis (A1).
 6. Thecutting insert (20) according to claim 1, wherein each radial plane (P1)forms an acute internal clearance angle (α) with a straight imaginaryclearance line (L_(C)) tangential to the clearance surface (34)immediately adjacent its respective first point (N1), and wherein theacute clearance angle (α) has a value equal to or greater than 75degrees.
 7. The cutting insert (20) according to claim 1, wherein eachof the three cutting portions (26) is radially spaced apart from eachcentral boss (36), and wherein all of the three cutting portions (26)are identical.
 8. The cutting insert (20) according to claim 1, whereina central bore (62) extends coaxially with the central axis (A1), andwherein the central bore (62) is visible in both end views of thecutting insert (20).
 9. The cutting insert (20) according to claim 1,wherein each of the three cutting portions (26) has exactly one majorcutting edge (30) and two minor cutting edges (40), and wherein eachminor cutting edge (40) extends away from its associated major cuttingedge (30) in a direction towards the median plane (M).
 10. The cuttinginsert (20) according to claim 1, wherein in an end view of the cuttinginsert (20), the entire rake surface (32) of each cutting portion (26)is located on the same side of its respective radial plane (P1) as theadjacent clearance surface (34).
 11. The cutting insert (20) accordingto claim 1, wherein the three major cutting edges (30) define a secondimaginary circle (C2) having a second diameter (D2) which circumscribesthe peripheral side surface (24), and wherein the center of the secondimaginary circle (C2) is coincident with the central axis (A1).
 12. Thecutting insert (20) according to claim 11, wherein in an end view of thecutting insert (20) a third imaginary circle (C3) contains the visiblecentral boss (36) and exactly two of the three cutting portions (26),wherein the second and third imaginary circles (C2, C3) have the samesecond diameter (D2), and wherein an offset distance (O) between thecenters of the second and third imaginary circles (C2, C3) is greaterthan a fifth of their second diameter (D2) and preferably equal to orgreater than a quarter of their second diameter (D2).
 13. The cuttinginsert (20) according to claim 1, wherein each central boss (36) hasthree second points (N2) contained in the three radial planes (P1) andeach second point (N2) is located the shortest distance between itsrespective central boss (36) and its respective first point (N1). 14.The cutting insert (20) according to claim 13, wherein each second point(N2) is located on a curved portion (76) of its respective central boss(36), and wherein each curved portion (76) is outwardly convex inrelation to the central axis (A1).
 15. The cutting insert (20) accordingto claim 1, wherein each end surface (22) includes a lower surface (48)extending radially inwardly from each of the three cutting portions(26).
 16. The cutting insert (20) according to claim 15, wherein eachlower surface (48) is axially closer to the median plane (M) than aportion of the major cutting edge (30) of its associated cutting portion(26).
 17. The cutting insert (20) according to claim 15, wherein eachlower surface (48) is substantially planar and perpendicular to thecentral axis (A1), and wherein each lower surface (48) defines a lowerplane (P2) which intersects the major cutting edge (30) of itsassociated cutting portion (26).
 18. A cutting tool (50) comprising aninsert holder (52), and at least one cutting insert (20) in accordancewith claim 1 retained therein, wherein exactly one cutting portion (26)of each cutting insert (20) is operative.
 19. The cutting tool (50)according to claim 18, wherein the insert holder (52) has a holdingportion (56) extending along a longitudinal tool axis (A2), the holdingportion (56) having a seating surface (58) transverse to thelongitudinal tool axis (A2), wherein one support surface (38) of eachcutting insert (20) is in clamping contact with the seating surface(58), and wherein in an end view of the cutting tool (50), only theoperative cutting portion (26) of each cutting insert (20) is locatedoutside the perimeter of the seating surface (58).
 20. The cutting tool(50) according to claim 19, wherein: each cutting insert (20) comprisesa central bore (62) extending coaxially with its central axis (A1), eachcutting insert (20) is removably securable to the insert holder (52) bymeans of a fastener (54), and the fastener (54) is in the form of aclamping screw (64) extending through the cutting insert's central bore(62) and engaging a threaded bore (66) in the seating surface (58) ofthe holding portion (56).
 21. The cutting tool (50) according to claim19, wherein the holding portion (56) is elongated and longitudinallyextends away from a shank portion (74) and exactly one cutting insert(20) is secured to the insert holder (52), wherein the three majorcutting edges (30) define a second imaginary circle (C2) having a seconddiameter (D2) which circumscribes its peripheral side surface (24), andwherein in an end view of the cutting tool (50) the second imaginarycircle (C2) contains the entire holding portion (56).
 22. The cuttingtool (50) according to claim 21, wherein in an end view of the cuttingtool (50) a third imaginary circle (C3) contains the entire holdingportion (56), and wherein the second and third imaginary circles (C2,C3) have the same second diameter (D2) and an offset distance (O)between the centers of the second and third imaginary circles (C2, C3)is greater than a fifth of their second diameter (D2) and preferablyequal to or greater than a quarter of their second diameters (D2). 23.The cutting tool (50) according to claim 21, wherein at least oneprotuberance (60 a, 60 b) protrudes from the seating surface (58), andwherein in an end view of the cutting tool (50), exactly two spacedapart corner portions (42 a, 42 b) of the at least one protuberance (60a, 60 b) are located inside a first imaginary circle (C1) circumscribingthe visible central boss (36) of the cutting insert (20).
 24. Thecutting tool (50) according to claim 23, wherein the peripheral sidesurface (24) of the cutting insert (20) has exactly three abutmentrecesses (28), and wherein exactly two of the three abutment recesses(28) are engaged with the exactly two spaced apart corner portions (42a, 42 b).